The present invention relates to a tapered roller bearing to be used in, for example, differential units, transmissions and the like.
In tapered roller bearings used in differential units, transmissions and the like of automobiles, preload given in assembly process is controlled by a torque under low-speed rotation. Large variations in this torque under low-speed rotation (assembling torque), if involved, could lead to failures such as earlier seizure due to excessively large preloads and rigidity deteriorations due to excessively small preloads.
Therefore, in order to give a proper preload to the tapered roller bearing, it is required that the assembling torque should less vary and less fluctuate.
The assembling torque of a tapered roller bearing arises, in most cases, from the friction between the inner-ring cone-back rib face and the roller large end face. Accordingly, surface roughness of the inner-ring cone-back rib face and the roller large end face, thickness of an oil film to be formed between the inner-ring cone-back rib face and the roller large end face, contact position between the rib face and the end face, and the like would largely affect the coefficient of friction, i.e., the torque.
As a technique of torque stabilization, a design in which the rib face and the roller end face are roughened has generally been adopted. Also, it is often the case that a rib roughness "sgr"1 and a roller-end-face roughness "sgr"2 are represented by a composite roughness "sgr" shown by the following equation (1):
"sgr"=("sgr"12+"sgr"22)xc2xdxe2x80x83xe2x80x83(1) 
where the assembling torque is controlled by this composite roughness "sgr".
However, there is a difference in the extent of effect on the assembling torque between the rib face and the roller end face. It has been found out by the inventors"" studies that assembling torque cannot be controlled enough only by means of the above composite roughness "sgr".
Also, since contact portion between the rib face and the roller end face varies in surface roughness and configuration due to friction as the operating time elapses, the preload on the tapered roller bearing decreases as compared with that at the start of operation. Further, the preload variation increases with increasing roughness of the rib face and the roller end face, i.e., increasing composite roughness.
Due to this, it has been difficult for conventional designs to achieve preload holding performance and constant assembling torque performance at the same time.
Meanwhile, the preload holding performance being high (the preload variation being small) is an important performance that is required by customers in addition to the variation in assembling torque being small.
Accordingly, an object of the present invention is to provide a tapered roller bearing which is capable of stabilizing the assembling torque and improving the preload holding performance.
In order to achieve the above object, there is provided a tapered roller bearing in which a large-end-face roughness "sgr"1 of a tapered roller is not less than 0.04 xcexcmRa;
a composite roughness "sgr", which is a square root of a sum between a square of the large-end-face roughness "sgr"1 and a square of an inner ring large-end-face roughness "sgr"2 that makes sliding contact with the large end face of the tapered roller, is set to be not more than 0.17 xcexcmRa;
a radius of curvature ratio R1/R2, which results from dividing a convex radius of curvature R1 of the large end face of the tapered roller by a concave radius of curvature R2 of the inner-ring large end face, is set to be not more than 0.35.
In the tapered roller bearing of this constitution, since the large-end-face roughness "sgr"1 of the tapered roller is set to be not less than 0.04 xcexcmRa, the rotating torque (assembling torque) becomes generally constant (average value: 1.00-1.18 Nxc2x7m) and smaller in fluctuation (0.13 Nxc2x7m at maximum) over a range that the cone-back rib face roughness "sgr"2 is 0.03-0.23 xcexcmRa (center-line average roughness) as shown in FIG. 4. Meanwhile, when the large-end-face roughness "sgr"1 is set to 0.02 xcexcmRa, torque variations are considerably large (maximum variation: 0.58 Nxc2x7m) so that the rotating torque varies over a range of 0.58-1.02 Nxc2x7m under the effects of the rib face roughness "sgr"2.
Also, since the composite roughness "sgr"=("sgr"12+"sgr"22){fraction (1/2 )} is set to be not more than 0.17 xcexcmRa, the preload retention rate on the regression curve can be made 90% or more as shown in FIG. 7.
Also, since the radius of curvature ratio R1/R2 resulting from dividing the convex radius of curvature R1 of the large end face of the tapered roller by the concave radius of curvature R2 of the cone-back rib face of the inner ring is set to be not more than 0.35, the rotating torque becomes smaller in variation (1.03-1.18 Nxc2x7m) and also smaller in fluctuations (0.13 Nxc2x7m at most) over a range that the composite roughness "sgr" is 0.05-0.22 xcexcmRa as shown in FIG. 6. Meanwhile, when the radius of curvature ratio R1/R2 is set to 0.69, larger than 0.35, the average value of rotating torque becomes lower (0.89 Nxc2x7m) and also its fluctuations become larger (0.40 Nxc2x7m at most) at a composite roughness "sgr"=0.05 xcexcmRa.
Further, even if the composite roughness "sgr" is kept generally equal as shown in FIG. 5, a change of the roller-end-face roughness "sgr"1 causes average value and fluctuations of the rotating torque to change so that the rotating torque cannot be controlled only by the composite roughness "sgr". That is, the roller-end-face roughness "sgr"1 has a larger effect on the torque under low-speed rotation than the rib-face roughness "sgr"2, and therefore the control of the roller-end-face roughness "sgr"1 is important for the stabilization of assembling torque.
Therefore, according to the tapered roller bearing of this constitution, the stabilization of assembling torque and the preload holding performance can be achieved at the same time.
In one embodiment of the present invention, the large-end-face roughness "sgr"1 of the tapered roller is set to be not more than 0.10 xcexcmRa;
the composite roughness "sgr" is set to be not more than 0.12 xcexcmRa; and
the radius of curvature ratio R1/R2 is set to be not less than 0.07.
In this embodiment, since the large-end-face roughness "sgr"1 of the tapered roller is set to be not less than 0.04 xcexcmRa and not more than 0.10 xcexcmRa, the rotating torque can be made to fall within a range of 1.00-1.11 Nxc2x7m in average value so that the rotating torque can be made even closer to a constant value, as shown in FIG. 4.
Also, since the composite roughness "sgr" is set to be not more than 0.12 xcexcmRa, the preload retention rate can be made to be 92% or more on a regression curve as shown in FIG. 7.
Also, since the radius of curvature ratio R1/R2 is set to be not less than 0.07 and not more than 0.35, the rotating torque becomes smaller in variation (1.03-1.18 Nxc2x7m) and also smaller in fluctuation (0.13 Nxc2x7m at most) over a range that the composite roughness "sgr" is 0.05-0.22 xcexcmRa as shown in FIG. 6.
Further, the radius of curvature ratio R1/R2 being not less than 0.07 means that the radius of curvature R2 of the cone-back rib face of the inner ring is not infinite, and that the cone-back rib face is not such flat as shown in FIG. 2 but such a concave curved surface as shown in FIG. 3. Therefore, an oil film is more easily formed between the cone-back rib face and the roller end face so that contact surface pressure also becomes lower. Thus, preload holding performance and seizure-resistant performance are improved.